Numerous gravity-type marine or river structures, i.e. they require their own sufficient weight to ensure the stability thereof against the actions to which they are subjected, achieve said stabilising weight by means of filling with granular materials, usually sand. This filling can be performed by mechanical means (pouring or placing with land or marine equipment) or by hydraulic means (by means of pumping a mixture of water and sand). However, once the filling of said structures has been completed and since this is an essential element for guaranteeing the functional nature of the structure, it is necessary to protect this filling from external actions that can cause this material to be lost. In structures wherein this filling stays enclosed by the very structure, this risk does not exist. However, some structures by their very design must remain completely or partially open, the filling being exposed to a greater or lesser degree to the actions of external agents. In the latter case and for marine structures, these agents are waves and currents. Meanwhile, in river structures the main agent is the current. In both cases, these agents can cause the filling material to be lost by means of the progressive scouring and transportation towards the outside.
Normally, if the structures have an emerged portion, the protection of this filling against these agents is simple, the use of diverse elements for the protection thereof being common: breakwaters, concrete blocks, bags of sand or concrete, gabions, in situ concrete slabs or even slabs that enclose the material by forming completely closed structures. When they are submerged gravity structures, the use of the same elements for protecting the filling as in emerged structures can be applied. However, the installation thereof may be more difficult to execute. Furthermore, in the case of progressive loss of filling material, the replacement thereof as well as the repair of these elements can be complex and expensive.
Another main constraint for these submerged structures is the difficulty of performing the filling. To do so and once the structure is supported on the bottom in the final location thereof, the structure must have one or several inlets to enable the introduction of filling material from the outside. Although the systems for executing said filling of the structure are the same as those described previously (mechanical or hydraulic) in many cases, by being submerged structures, there are difficulties in accessing by land means, the use of marine means usually being necessary. Likewise, it is common to use hydraulic methods for performing said filling, by using dredges that pump the fluid (mixture of water and sand) into the element. However, a contradiction is then produced between the constructive complexity (which requires openings that are large enough to introduce the filling inside the structure) and the capacity of the structure itself to provide protection for the filling on the inside.